Relative stabilities of three low-energy tautomers of cytosine: A coupled cluster electron correlation study

High-level quantum chemical calculations have been carried out in an effort to reinvestigate the relative stabilities of the three lowest-lying tautomers of cytosine. Geometries were optimized at the CCSD/TZP level and electronic energies calculated at CCSD(T)/cc-pVTZ and vibrational frequencies at MP2/TZP. Comparative DFT calculations were also performed. From these data Gibbs free energies and equilibrium mole ratios were calculated. In agreement with most previous theoretical and experimental results, the aminohydroxy tautomer 2b was found to be the most stable structure. As a new result, the amino-oxo form I and the imino-oxo form 3a have very nearly the same electronic energy, about 1.5-1.7 kcal/mol above 2b. The calculated DeltaG values at standard temperature are similar to0.8 kcal/mol relative to 2b, again for both I and 3a. These results about the stability of the oxo form I are in quantitative agreement with experimental estimates in the literature, both from matrix isolation infrared and from molecular beam microwave spectroscopy. However, the calculated stability of the imino form is much higher than suggested by experiment. Our tentative reanalysis of the IR spectrum did not resolve this discrepancy. The widely used MP2 method gives significant deviations from the coupled cluster results and may be not accurate enough for determining close-lying energies of tautomers. Also, density functional theory gives qualitatively different results from traditional wave function methods.